The present invention relates to a method for bit error rate measurements in a cell-based telecommunication system.
In cell-based telecommunication systems, the information is transferred by means of cells of fixed or variable byte-length. These cells typically have an overhead section wherein control, management and routing information is embedded and a payload section, carrying user information.
Cell-based telecommunication systems comprise for example ATM (Asynchronous Transfer Mode) communication networks. ATM cells for instance have a fixed length of 53 bytes, 8 bytes of which constitute the overhead section or the so called ATM cell header, reserved for overhead information.
The whole transmitted cell-stream in a cell-based network is generally composed of two different types of cells: on the one hand used cells carrying in their payload section the proper user information and on the other hand unused cells with no user information in their payload section. The unused cells are generated and inserted amid the used cells to be transmitted in order to provide a continuous cell-stream between transmitting means and receiving means.
Bit errors which occur during transmission of these cells, have a negative impact onto the quality of the connection. A bit error which occurs in the payload section of the cell implies a falsification of the transmitted information; wherein a bit error which occurs in the overhead section of the cell potentially implies a falsification of the destination address, and thus a loss of the cell.
Therefore, in cell-based telecommunication systems, the bit error rates (ratio of bits transferred erroneously due to noise or impairments of the physical transmission medium in proportion to the totality of transferred bits) are traditionally measured during initialisation of the system or during a test phase. Hereby a predetermined signal is sent from a transmitter to a receiver. The arriving signal is then analysed at the receiver's side by comparing the transmitted signal with a generated reference signal.
In an ADSL (Asymmetric Digital Subscriber Line) system for instance, the signal to noise ratio (SNR) is measured at the different carrier frequencies during initialisation of the system. This signal to noise ratio can be seen as a measure for the expected bit error rate. The results of these measurements afterwards are used to determine the bit allocations, i.e. the way wherein data bits are distributed over the different carriers that constitute a DMT (Discrete Multi Tone) symbol, to cope with the maximum allowable bit error rate of 10−7 prescribed by the ADSL standard. The SNR measurements and the use thereof in the bit allocation process are described in the ANSI (American National Standards Institute) ADSL Standard T1E1.4, in paragraph 12 entitled ‘Initialization’ on pages 87-111 and in paragraph 6.5 entitled ‘Tone Ordering’ on Pages 36-37.
Traditional bit error rate measurement methods have the disadvantage, that normal operation of the system has to be interrupted and the system has to be brought in a test phase. During this test phase, no transmission of proper user information is possible.